1. Field of the Invention
The present invention relates generally to a scheduling apparatus and method in a communication system, and in particular, to an apparatus and method for uplink scheduling in a communication system.
2. Description of the Related Art
Providing services with diverse Quality of Service (QoS) levels at about 100 Mbps to users is actively being studied for a 4th Generation (4G) communication system, which is a future-generation communication system. Particularly, there is ongoing research concerning the development of a new communication system that supports high-speed services by ensuring mobility and QoS to a Wireless Local Area Network (WLAN) supporting relatively high data rates and a Wireless. Metropolitan Area Network (WMAN).
Institute of Electrical and Electronics Engineers (IEEE) 802.16a and IEEE 802.16e communication systems adopt Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access (OFDM/OFDMA) for the physical channels of the WMAN system in order to support a broadband transmission network. Because a physical channel signal is transmitted on a plurality of sub-carriers in OFDM/OFDMA in the WMAN system, IEEE 802.16a and IEEE 802.16e system enable high-speed data transmission. In effect, IEEE 802.16a and IEEE 802.16e are OFDM/OFDMA-Broadband Wireless Access (BWA) communication systems.
IEEE 802.16a considers only a single-cell structure without regards to the mobility of Subscriber Stations (SSs). In contrast, IEEE 802.16e supports the mobility of the SS to the IEEE 802.16a communication system. For the IEEE 802.16e system to ensure the mobility of SSs in a multi-cell environment, the operations of the SS and the Base Station (BS) must be modified. There is ongoing study, particularly regarding handover for the SS in the multi-cell structure. Hereinafter, a Mobile SS is called an MS.
FIG. 1 illustrates the configuration of a typical BWA communication system. Referring to FIG. 1, the BWA communication system is configured in a multi-cell structure. Thus, it includes cells 100 and 150, a BS 110 that covers the cell 100, a BS 140 that covers the cell 150, and a plurality of MSs 111, 113, 130, 151 and 153. Signaling between the BSs 110 and 140 and the MSs 111, 113, 130, 151 and 153 is based on OFDM/OFDMA.
In OFDMA provided for the BWA communication system, subchannels include subcarriers, which constitute one OFDM symbol, and a plurality of OFDM symbols form one frame.
FIG. 2 illustrates a structure of a data frame in the typical BWA communication system. Particularly, the data frame is an UpLink (UL)/DownLink (DL) frame for an Orthogonal Frequency Division Multiple Access-Time Division Duplex (OFDMA-TDD) BWA communication system.
Referring to FIG. 2, the data frame is divided into a DL frame and a UL frame in time. A transmission gap, called a Transmit/receive Transition Gap (TTG), is interposed for transition from the downlink to the uplink, and a transmission gap, called a Receive/transmit Transition Gap (RTG), is interposed for transition from the uplink to the downlink. The horizontal axis represents OFDMA symbol numbers and the vertical axis represents the logical number of subchannels.
On the downlink, a preamble resides in a Kth OFDMA symbol, for synchronization acquisition, and data information broadcast most common to MSs such as a Frame Control Header (FCH) and a DL-MAP is located in a (K+1)th OFDMA symbol.
The preamble delivers a synchronization signal, i.e. a preamble sequence by which synchronization is acquired between a BS and an MS. That is, the preamble is required for the MS to acquire synchronization to data transmitted by the BS. The MS extracts the synchronization information from the preamble through its MODEM.
The FCH includes two subchannels, carrying basic information about subchannels, ranging, and modulation. An analysis of the FCH indicates the size of the DL-MAP and the frequency reuse factor (referred to as reuse) of the BS, for example, reuse 1 or reuse 2.
The DL-MAP delivers a DL-MAP message. The DL-MAP message has information required for extracting data and providing service provisioning to MSs, such as the positions and sizes of data in the DL frame. Therefore, data can be extracted from the DL frame by analyzing the DL-MAP information.
In (K+3)th through (K+15)th OFDMA symbols, DownLink bursts (DL bursts) are positioned, for example, as DL burst #1 to DL burst #6. Data is extracted from the DL bursts based on general data information, such as information acquired from the DL-MAP.
On the uplink, UpLink bursts (UL bursts), for example UL burst #1 to UL burst #5, are located in (K+17)th to (K+26)th OFDMA symbols. A Ranging Subchannel also occupies the (K+17)th to (K+26)th OFDMA symbols. Ranging subchannels for ranging are transmitted in the Ranging Subchannel. Data is extracted from the UL bursts based on general data information such as information acquired from an UpLink MAP (UL-MAP), which is delivered in DL burst #1.
Each subchannel is composed of a plurality of subcarriers. Depending on system conditions, one subchannel is composed of a predetermined number of subcarriers. As described above, the IEEE 802.16e OFDMA communication system configures subchannels each being a set of subcarriers according to system condition and allocates resources to a plurality of users (i.e., MSs) by the subchannels.
FIG. 3 illustrates traffic transmission in the typical BWA communication system. Particularly, a BS 350 allocates uplink resources to, for example, an MS 320 and the MS 320 transmits traffic using the allocated resources.
Referring to FIG. 3, the BS 350 selects an MS (e.g., the MS 320) to allocate resources to from among MSs 310, 320 and 330 within its coverage area by a predetermined scheduling procedure. The BS 350 determines the amount of resources and a Modulation and Coding Scheme (MCS) for the MS 320. Also, the BS 350 determines the position of the resources by which the MS 320 transmits data. This operation is repeated until all uplink resources are allocated.
After scheduling is completed, the BS 350 broadcasts uplink scheduling information indicating the result of the scheduling in a UL-MAP message to all of the MSs 310, 320 and 330 within a cell.
The MSs 310, 320 and 330 receive the UL-MAP message and, if resources have been allocated to them, they transmit traffic at the positions of the allocated resources on the uplink. For example, if the UL-MAP message indicates the existence of resources allocated to the MS 320, then the MS 320 transmits uplink traffic at the position of the allocated resources.
Basically, the BS seeks to maximize radio channel throughput and minimize the transmission delay of traffic in uplink scheduling. Besides, the BS aims to reduce interference with neighboring sectors, reduce UL-MAP overhead, expand cell coverage, reduce the power consumption of the MSs, and maintain stable link performance between the BS and the MSs. Thus, the BS selects an operation method for achieving the above objectives. However, some objectives of the BS scheduling may contradict each other. Hence, the scheduler of the BS attempts to fulfill the objectives appropriately according to their priority levels by adjusting parameters like an MCS level preference and the amount of resources allocated per time.
Yet, there is no specified scheduling method and apparatus for satisfying the above various objectives and that flexibly using weights, in allocating uplink resources through frame-basis uplink scheduling in a broadband mobile communication system, particularly an OFDMA-TDD mobile communication system.